This invention is related to the sustained delivery of a biologically active agent. More particularly, the invention is directed to an implantable delivery system for the prolonged delivery of an active agent to a fluid environment in a natural or artificial body cavity.
Treatment of disease by prolonged delivery of an active agent at a controlled rate has been a goal in the drug delivery field. Various approaches have been taken toward delivering the active agents.
One approach involves the use of implantable diffusional systems. For example, subdermal implants for contraception are described by Philip D. Darney in Current Opinion in Obstetrics and Gynecology 1991, 3:470-476. Norplant(copyright) requires the placement of 6 levonorgestrel-filled silastic capsules under the skin. Protection from conception for up to 5 years is achieved. The implants operate by simple diffusion, that is, the active agent diffuses through the polymeric material at a rate that is controlled by the characteristics of the active agent formulation and the polymeric material. Darney further describes biodegradable implants, namely Capranar(trademark) and norethindrone pellets. These systems are designed to deliver contraceptives for about one year and then dissolve. The Capranor(trademark) systems consist of poly(xcex5-caprolactone) capsules that are filled with levonorgestrel and the pellets are 10% pure cholesterol with 90% norethindrone. Implantable infusion pumps have also been described for delivering drugs by intravenous, intra-arterial, intrathecal, intraperitoneal, intraspinal and epidural pathways. The pumps are usually surgically inserted into a a subcutaneous pocket of tissue in the lower abdomen. System""s for pain management, chemotherapy and insulin delivery are described in the BBI Newsletter, Vol. 17, No. 12, pages 209-211, December 1994. These systems provide for more accurately controlled delivery than simple diffusional systems.
One particularly promising approach involves osmotically driven devices such as those described in U.S. Pat. Nos. 3,987,790, 4,865,845, 5,057,318, 5,059,423, 5,112,614, 5,137,727, 5,234,692 and 5,234,693 which are incorporated by reference herein. These devices can be implanted into an animal to release the active agent in a controlled manner for a predetermined administration period. In general, these devices operate by imbibing fluid from the outside environment and releasing corresponding amounts of the active agent.
The above-described devices have been useful for delivering active agents to a fluid environment of use. Although these devices have found application for human and veterinary purposes, there remains a need for devices that are capable of delivering active agents, particularly potent unstable agents, reliably to a human being at a controlled rate over a prolonged period of time.
Implantable osmotic systems for delivery of an active agent to an animal are well known. Adaptation of these systems for human use raises a number of difficult issues. The size of the device may need to be decreased for human implantation. The strength of the device must be sufficient to ensure a robust system. Accurate and reproducible delivery rates and durations must be ensured and the period from implantation to start-up of delivery must be minimized. The active agent must return its purity and activity for extended periods of time at the elevated temperatures encountered in the body cavity.
Accordingly, in one aspect, the invention is a fluid-imbibing device for g delivering an active agent formulation to a fluid environment of use. The device comprises a water-swellable, semipermeable material that is received in sealing relationship with the interior surface at one end of an impermeable reservoir. The device further contains an active agent to be displaced from the device-when the water-swellable material swells.
In another aspect, the invention is directed to an implantable device for delivering an active agent to a fluid environment of use. The device comprises a reservoir and a back diffusion regulating outlet in a mating relationship. The flow path of the active agent comprises a pathway formed between the mating surfaces of the back diffusion regulating outlet and the reservoir.
In yet another aspect, the present invention is directed to a device for storing an active agent in a fluid environment of use during a predetermined administration period, the device comprising a reservoir containing an active agent The reservoir is impermeable and formed at least in part from a metallic material. The portion of the reservoir contacting the active agent is non-reactive with the active agent, and is formed of a material selected from the group consisting of titanium and its alloys.
In a further aspect, the invention is an implantable fluid-imbibing active agent delivery system that comprises an impermeable reservoir. The reservoir contains a piston that divides the reservoir into an active agent containing chamber and a water-swellable agent containing chamber. The active agent containing chamber is provided with a back-diffusion regulating outlet. The water-swellable agent containing chamber is provided with a semipermeable plug. Either the plug or the outlet is releasable from the reservoir at an internal pressure that is lower than the maximum osmotic pressure generated by the water-swellable agent.
The invention is further directed to a fluid-imbibing implantable active agent delivery system where the time to start-up of delivery is less than 10% of the predetermined administration period.
In another aspect, the invention is directed to a method for preparing a fluid-imbibing implantable active agent delivery system. The method comprises injection molding a semipermeable plug into the end of an impermeable reservoir such that the plug is protected by the reservoir.
In still another aspect, the invention is directed to an impermeable active agent delivery system for delivering an active agent that is susceptible to degradation. The reservoir contains a piston that divides the reservoir into a water-swellable agent chamber and an active agent chamber. The open end of the water-swellable agent chamber contains a semipermeable membrane and the open end of the active agent chamber contains a back-diffusion regulating outlet. The system effectively seals the active agent chamber and isolates it from the environment of use.
In a further aspect, the invention is directed to a back-diffusion regulating outlet useful in an active agent delivery system. The outlet defines a flow path wherein the length, interior cross-sectional shape and area provide for an average linear velocity of active agent that is higher than the linear inward flow of fluid in the environment of use.
The invention is also directed to a semipermeable plug useful in an active agent delivery system. The plug is water-swellable and must expand linearly in the delivery system to commence pumping upon insertion of the system into the fluid environment of use.
The invention is further directed to implantable delivery systems useful for delivering leuprolide.